Investigating the causes of foam formation in deep pit swine manure storages

The appearance of foam on the surface of deep pit swine manure storages throughout the United States and Canada is a serious concern for the pork industry. In addition to logistical issues caused by foam accumulation in deep pits, manure foam has the capacity to trap gases produced by the anaerobic decomposition of manure, leading to dangerous flammable gas concentrations upon agitation or foam disturbance. Recent flash fires and explosions at several swine facilities have created a pressing need to understand and mitigate the causes of foaming deep pits.\nA number of hypotheses regarding the contributing factors of foam formation exist. These include the presence of surface active agents (proteins, volatile fatty acids, detergents, lipids, biosurfactants, etc.) that enable foam generation, the presence of hydrophobic solids that stabilize foam, the presence of certain feed components that may contribute to the foaming mechanism, the rate of biogas production from the waste, or any combination of these and other physical, chemical, and microbiological characteristics of the manure slurry. Despite a broad understanding of the aspects of foaming systems, there is a lack of knowledge of the mechanism that causes foam to stabilize in anaerobic environments. The objective of this research was to better understand foaming manure systems from a "three-phase system" approach; that is, to research the solid, liquid, and gas phases of swine manure in deep pits and how they contribute to stabilized foam. Foam mitigation strategies were considered after these aspects of foaming systems were investigated.\nThe two studies presented in this thesis included extensive analysis of swine manure sampled from both commercial deep pits and controlled dietary studies. The first study was a field study, with manure samples collected from over 50 swine facilities in Iowa over 13 months. These samples were analyzed for a number of parameters including temperature, pH, total and volatile solids, short-chain and long-chain fatty acid concentration, biochemical methane potential, methane production rate, surface tension, foaming capacity index, and foam stability. An extensive database was compiled so that these parameters could be compared based on the extent of foam accumulation at the sampling site. The second study involved a controlled dietary study, where the impact of carbohydrate and protein sources on foaming parameters was measured. The results allowed us to understand the direct impact of feed components on swine manure parameters, as well as to compare the manure collected from these controlled trials to samples taken from commercial deep pits.\nAs a whole, these studies showed that swine manure from barns with foam accumulation exhibited significantly different trends in many biological and physicochemical parameters when compared to manure from non-foaming barns. Most notably, the rate at which biogas was generated in foaming barns was much greater than in non-foaming barns, indicating a much greater presence of the "gas phase" in foaming barns. At the same time, non-foaming barns showed a greater potential for cumulative biogas production and larger concentrations of important substrate such as short-chain fatty acids. Taken together, we see that the microbial consortium in foaming barns allows them to function as more efficient digesters, producing a greater methane flux through foaming pits, which is the driving force of foam formation. Manure collected from the surface of foaming systems was also able to produce foam at a greater capacity in laboratory tests when compared to manure from non-foaming systems, and also showed an enhanced ability to stabilize in the testing apparatus. These tests and others indicated an accumulation of a surfactant and/or stabilizing agent at the surface of foaming barns which promotes foam generation and stability. Other important differences in the temperature trends, pH, solids profiles, and surface tension measurements led to a greater understanding of the behavior of foaming swine manure deep pits. Overall, this knowledge can lead to more directed solutions, specifically in exploring sustainable ways to control the activity of the microbial community within deep pits.

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